THE UPPER ATMOSPHERE DOBSON 193 



of the strength and direction of the magnetic force of the earth 

 show that this force is not constant but varies both in strength and 

 direction. These variations, which are only small and require 

 delicate instruments to show them, can be divided into two quite 

 distinct classes. In the first class are the regular diurnal and annual 

 variations, while in the second are much larger irregular fluctua- 

 tions which occur occasionally and which, when very large, are 

 known as magnetic storms. While the main permanent magnetism 

 of the earth appears to have its origin within the earth, these varia- 

 tions are due to currents very high above the earth's surface. 



One of the simplest magnetic elements to measure is the declina- 

 tion, or the angle between the geographical and magnetic north. A 

 sensitive compass needle can easily be made by suspending a bar 

 magnet by a single thread so that it is quite free to turn in any di- 

 rection. If a small mirror be attached to it and a beam of light re- 

 flected by the mirror on to a scale, very small movements of the 

 magnet can be seen. With such an instrument it can be shown that 

 on many days the magnet goes through a regular movement which 

 is repeated each day, reaching a maximum in one direction in the 

 morning and a maximum in the other direction in the afternoon, 

 while it moves but little through the night. Such days are known 

 as magnetically quiet days. The total movement of the magnet is 

 quite small, being only about a sixth of a degree in summer and 

 only about one-twentieth of a degree in winter in England. If other 

 characteristics of the earth's magnetic field are measured, such as 

 the total intensity of the field or the angle between its direction and 

 a horizontal surface, the same type of effects will be observed. 



On other days there will be pronounced irregular variations of 

 the magnetic force which affect the whole world. These latter fluc- 

 tuations are much stronger in polar regions than in low latitudes, 

 and there is now little doubt that they owe their origin to streams 

 of charged particles shot out from the sun. The reason that they 

 are felt more in high latitudes is that the magnetic field of the earth 

 deflects the charged particles so that they strike the earth only in 

 the higher latitudes, where they produce a visible effect, namely, 

 the aurora. 



Both the regular and irregular variations of the earth's magnetic 

 field are connected with sun spots. Sun spots are dark markings on 

 the surface of the sun which can easily be seen with a small tele- 

 scope, while a large spot may occasionally be seen with the eye. The 

 exact nature of sun spots is at present unknown, but they indicate 

 in some way the activity of the sun, and go through a fairly regu- 

 lar cycle of about 11 years length. Thus, in some years there may 

 be a large number of spots so that nearly every day one or more 

 spots can be seen. Then the number will decrease during the next 



